In power transmission networks alternating current (AC) power is converted to direct current (DC) power for transmission via overhead lines, under-sea cables, underground cables, and so on. This conversion to DC power removes the need to compensate for the AC capacitive load effects imposed by the power transmission medium, i.e. the transmission line or cable, and reduces the cost per kilometre of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance. A converter, such as a voltage source converter, provides the required conversion between AC power and DC power within the network.
A typical such power transmission network includes AC-DC converters, which are connected via a DC transmission link and are arranged to interconnect an AC electrical system and a DC electrical network.
According to a first aspect of the invention, there is provided a power transmission network comprising a single-phase or multi-phase AC electrical system, a converter including at least one AC terminal, a point of common coupling, a phase reactance connecting the point of common coupling to the or each AC terminal of the converter, and at least one power transmission medium to interconnect the point of common coupling and the AC electrical system, wherein the power transmission network further includes a converter controller programmed to: process the voltage and current at the point of common coupling to compute a state vector corresponding to the power transmission network; derive a converter demand by combining the computed state vector with a plurality of control parameters, wherein the plurality of control parameters includes the capacitance of the power transmission medium or media presented at the point of common coupling and the impedance of the phase reactance; and operate the converter in accordance with the converter demand to control the converter voltage at the or each AC terminal and/or the voltage at the point of common coupling so as to inhibit any perturbation in the converter voltage from a target converter voltage or range resulting from the interaction between the capacitance of the power transmission medium or media and the impedance of the phase reactance.
The inventors have found that the interaction between the capacitance of the power transmission medium or media and the impedance of the phase reactance can result in a disturbance that adversely affects the ability of the converter to stably control the voltage at the or each AC terminal and/or the voltage at the point of common coupling at a desired voltage level or within a desired voltage range.
The provision of the converter controller to operate the converter therefore enables stable control over the converter voltage at the AC terminal and/or the voltage at the point of common coupling in the event of any perturbation in the converter voltage from a target converter voltage or range resulting from the interaction between the capacitance of the power transmission medium or media and the impedance of the phase reactance. This in turn stabilises the operation of the power transmission network as a whole.
In addition, the use of full state feedback control principles by the converter controller enables stable control over the converter voltage at the AC terminal and/or the voltage at the point of common coupling in the power transmission network over a wide range of frequencies.
Moreover, the capability of the converter controller to use the full state feedback control principles on the basis of the voltage and current at the point of common coupling minimises the need for a plurality of communications links laid throughout the power transmission network in order to obtain information about the power transmission network required to compute the state vector.
The configuration of the power transmission network may vary depending on its operating requirements.
The converter controller may be programmed to process measurements of the voltage and current at the point of common coupling or predicted values of the voltage and current at the point of common coupling.
The converter controller may be, but is not limited to, a linear controller, a discrete controller, or a digital controller.
The converter may be an AC-DC converter or a DC-DC converter.
The AC electrical system may include at least one AC power element in the form of a power source, e.g. a variable power source. The variable power source may be a renewable power source, such as a wind farm.
The AC electrical system may be an AC electrical network that includes a plurality of AC power elements, such as one or more sources, one or more loads and one or more other power transmission media. Optionally each of the plurality of AC power elements may be separately connected to the point of common coupling via a respective one of the power transmission media.
The phase reactance may include a transformer connected between the point of common coupling and the or each AC terminal of the converter. The phase reactance may include one or more other reactances placed in the AC path of the converter, such as valve reactors used in the converter
When a plurality of power transmission media interconnects the point of common coupling and the AC electrical system, the capacitance of the power transmission medium or media presented at the point of common coupling may be half of the total lumped capacitance of the power transmission medium or media.
The plurality of control parameters may be a matrix or vector of control parameters. The converter controller may be programmed to derive the converter demand by multiplying the computed state vector with the matrix or vector of control parameters to obtain a single value and by comparing the single value against a reference value to obtain a differential value that forms the converter demand.
The converter controller may be programmed to perform linear quadratic regulation (LQR) to derive the plurality of control parameters. The use of LQR allows the definition of the plurality of control parameters to provide an optimised robust response to any perturbation in the converter voltage from a target converter voltage or range resulting from the interaction between the capacitance of the power transmission medium or media and the impedance of the phase reactance.
The converter controller may be programmed to perform a three phase stationary reference frame to direct/quadrature rotating reference frame transformation (abc-to-dq) of the computed state vector prior to its combination with the plurality of control parameters to derive the converter demand. The use of an abc-to-dq transformed state vector by the converter controller further enhances the control over the converter voltage at the or each AC terminal and/or the voltage at the point of common coupling, particularly when such use is combined with the performance of linear quadratic regulation to derive the plurality of control parameters.
The power transmission network may further include an inductive or capacitive VAR compensator. When the VAR compensator is a capacitive VAR compensator, the capacitance of the VAR compensator is in an embodiment lower than the capacitance of the power transmission medium or the summed capacitance of the power transmission media.
The inclusion of an inductive VAR compensator minimises any detrimental effect a surge of power in the power transmission medium or media may have on the insulation associated with the power transmission medium or media. Meanwhile the inclusion of a capacitive VAR compensator minimises any adverse effect a variation in performance of the power transmission network may have on the stability of the control of the converter voltage at the or each AC terminal and/or the voltage at the point of common coupling.
The converter controller may be programmed to modify the plurality of control parameters in response to a change in the capacitance of the power transmission medium or the summed capacitance of the power transmission media. This may be done by modifying the plurality of control parameters in accordance with the states of circuit breakers associated with the power transmission media and/or in accordance with any changes in the performance of the power transmission network.
The converter controller may be programmed to process a proportional servo gain when deriving the converter demand, the value of the proportional servo gain being set to control the short circuit level of the AC electrical system so as to stabilise the voltage of the AC electrical system. The ability of the converter controller to operate the converter to stably control the converter voltage at the or each AC terminal and/or the voltage at the point of common coupling permits the use of a servo gain that is set at a sufficiently high value to achieve a desired short circuit level of the AC electrical system so as to stabilise the voltage of the AC electrical system, without the servo gain adversely affecting the stability of the power transmission network.
In such embodiments, the converter controller may be configured to use proportional feedback to set the value of the proportional servo gain to control the short circuit level of the AC electrical system. The proportional feedback may include a low pass filter.
According to a second aspect of the invention, there is provided a method of controlling a power transmission network, the power transmission network comprising a single-phase or multi-phase AC electrical system, a converter including at least one AC terminal, a point of common coupling, a phase reactance connecting the point of common coupling to the or each AC terminal of the converter, and at least one power transmission medium to interconnect the point of common coupling and the AC electrical system, wherein the method includes the steps of: process the voltage and current at the point of common coupling to compute a state vector corresponding to the power transmission network; derive a converter demand by combining the computed state vector with a plurality of control parameters, wherein the plurality of control parameters includes the capacitance of the power transmission medium or media presented at the point of common coupling and the impedance of the phase reactance; and operate the converter in accordance with the converter demand to control the converter voltage at the or each AC terminal and/or the voltage at the point of common coupling so as to inhibit any perturbation in the converter voltage from a target converter voltage or range resulting from the interaction between the capacitance of the power transmission medium or media and the impedance of the phase reactance.
The features and corresponding advantages of the first aspect of the invention applies mutatis mutandis to the second aspect of the invention.